A transmission electron microscope (TEM) creates images by generating an electron beam that penetrates a very thin specimen. A projected image of electron intensity corresponds to the specimen structure. Direct exposure of photographic emulsion on either glass plates or film has historically produced TEM images with the highest resolution and information density. Electronic camera systems that view screens coated with phosphors.sup.1 or scintillators.sup.2 are often used in recording when photographic film is not needed or when high sensitivity and live time viewing are required. When coupled to digital image acquisition systems, electronic cameras provide an cost efficient way to store images on digital media for viewing, digital analysis, and archival. FNT .sup.1 G. F. J. Garlick, "Cathodo- and Radioluminescence", in Luminescence of Inorganic Solids, edited by P. Goldberg, Academic Press (1966) pp. 685-731. FNT .sup.2 R. Autrata, P. Schauer, Jos. Kvapil, "Single-Crystal Aluminates--A New Generation of Scintillators for Scanning Electron Microscopes and Transparent Screens in Electron Optical Devices", Scanning Electron Microscopy, Pt. 2, (1983) pp. 489-500.
The resolution available from an electronic camera system in a TEM is limited by the phosphor or scintillator screen. Since the highest available resolution is about 25 line-pairs/millimeter (i.e., lp/mm), camera systems must typically have relatively large, bulky screens. The large screens must, in turn, be optically coupled to either 1) large, expensive camera sensors at unity magnification or 2) smaller, more economical sensors at as much as 3:1 demagnification. Unity magnification systems using either fiber optics or lens optics can provide detection quantum efficiency of close to 100%, but are inherently expensive. Demagnifying systems are less efficient than unity magnification systems.sup.6. In addition, the modulation transfer function (MTF) at this resolution is low so that faint edges often have poor visibility. FNT .sup.6 I. P. Csorba, Image Tubes, Howard W. Sams & Co., Inc., (1985).
In all systems operating below 1 MeV, a phosphor or scintillator with a thickness of typically 40 microns or less is deposited on a light transparent support structure, such as a glass plate or a fiber optic bundle. Electrons first excite the phosphor so that it emits visible light. Then they are absorbed by the glass beneath the phosphor. The visible light image is transmitted to an electronic camera through a transparent support material via a lens system.sup.3 or a fiber optic bundle.sup.4. The best resolution for these systems (i.e., about 25 lp/mm) is achieved at around 100 keV. FNT .sup.3 S. Matsuura, T. Nino, K. Oba, and S. Horiuchi, "Development of High Sensitive Imaging Device for TEM", Proc. XI.sup.th Cong. on Electron Microscopy, Kyoto (1986) pp. 441-442. FNT .sup.4 B. Kraus, O. L. Krivanek, N. T. Swann, C. C. Ahn, and P. R. Swann, "Performance of Newvicon and CCD Real-Time EM Observation Systems", Proc. XI.sup.th Cong. on Electron Microscopy, Kyoto (1986) pp. 455-456.
At 1 million electron volts (1 MeV) Hermann.sup.5 found that radiation damage precluded the use of fiber optics, and he employed plastic membranes with a thick phosphor layer and a tandem lens at unity magnification to achieve sufficient brightness. Due to his use of a thick phosphor, the resolution achieved by his screens was poor (i.e., less than 20 lp/mm). FNT .sup.5 K.-H. Hermann, "Electron Image Conversion," Proc. Elect. Micr. Soc. Am. (1986) pp. 78-81.